1. Field of the Invention
The present invention relates to resinous thermoplastic blend compositions suitable for employment in the fabrication of thermoplastic films and, in particular, oriented thermoplastic films such as oriented polypropylene or copolymers thereof with ethylene. Certain uncoated, oriented films such as polypropylene and ethylene-propylene copolymers have relatively high heat sealing temperatures and narrow heat sealing ranges in contrast to other available thermoplastic films such as polyethylene, for example. Additionally, such oriented films exhibit a tendency to disorient, shrink, and tear when attempts are made to heat seal them at their prerequisitely high heat sealing temperature.
In accordance with the present invention, a minor amount (e.g. from about 5 percent to about 40 percent by weight) of a normally solid resin interpolymer of pentadiene with at least one other hydrocarbon having ethylenic unsaturation copolymerized therewith, is blended with, for example, polypropylene resin prior to extrusion of the polypropylene into a film structure. Specific embodiments of such normally solid resins include a multipolymer of a mixture of pentadiene, isoprene, cyclopentadiene and at least one of the following: 2-methylbutene-2, t-butylstyrene, .beta.-pinene, .beta.-phellandrene, dipentene, .alpha.-methylstyrene and mixtures thereof; and still more specifically, such multipolymers derived from a mixture comprising pentadiene and 2-methylbutene-2 as its major components (and preferably in about equal parts by weight); mixtures thereof with t-butylstyrene, or with beta-pinene, or with a mixture of alpha methyl styrene, dipentene and beta-phellandrene. Oriented polypropylene film structures prepared from such resinous blend mixtures have been found to exhibit substantially lower minimum heat seal temperatures, thereby broadening its heat seal range. The interpolymer containing film has improved optical properties and a significantly improved, or higher, tensile modulus or stiffness thereby improving the film's processability on automatic packaging equipment.
The heat sealing characteristics of the novel film compositions of the present invention may be further improved by subjecting such films to electronic treatment such as corona discharge treatment utilizing techniques which are well known in the art.
2. Description of the Prior Art
U.S. Pat. No. 3,278,646 describes the employment of terpene polymers, such as .beta.-pinene for example, in additive amounts to improve the heat seal characteristics of oriented polypropylene film. However, such blends have, in instances, been found to have poor surface properties, increased blocking, narrow sealing ranges and poor dimensional stability. Other attempts to remedy the heat sealing difficulties of such oriented films include providing the film with a coating of material that is more heat sensitive and hence sealable at a lower temperature than the film itself. However, such coating techniques are both time consuming and costly, involving the formulation of special coating materials, special treatment of the film surface to insure coating adhesion as well as necessitating the employment of film coating equipment. Additionally, in many instances such coatings have been found to have an adverse effect on film physical properties such as increasing the blocking tendency of the coated films, reducing the film optical properties, and causing higher coefficients of friction.